.TH std::packaged_task::packaged_task 3 "2024.06.10" "http://cppreference.com" "C++ Standard Libary"
.SH NAME
std::packaged_task::packaged_task \- std::packaged_task::packaged_task

.SH Synopsis
   packaged_task() noexcept;                                          \fB(1)\fP \fI(since C++11)\fP
   template< class F >                                                \fB(2)\fP \fI(since C++11)\fP
   explicit packaged_task( F&& f );
   template< class F, class Allocator >                                   \fI(since C++11)\fP
   explicit packaged_task( std::allocator_arg_t, const Allocator& a,  \fB(3)\fP \fI(until C++17)\fP
   F&& f );
   packaged_task( const packaged_task& ) = delete;                    \fB(4)\fP \fI(since C++11)\fP
   packaged_task( packaged_task&& rhs ) noexcept;                     \fB(5)\fP \fI(since C++11)\fP

   Constructs a new std::packaged_task object.

   1) Constructs a std::packaged_task object with no task and no shared state.
   2,3) Constructs a std::packaged_task object with a shared state and a copy of the
   task, initialized with std::forward<F>(f).
   The allocator a is used to allocate memory necessary to store the task.
   \fI(until C++17)\fP
   These overloads participate in overload resolution only if std::decay<F>::type is
   not the same type as std::packaged_task<R(Args...)>.

   Let t1, t2, ..., tN be values of the corresponding types in Args, if
   INVOKE<R>(f, t1, t2, ..., tN) is not a valid expression, the program   \fI(until C++17)\fP
   is ill-formed.
   If std::is_invocable_r_v<R, F, Args...> is false, the program is       \fI(since C++17)\fP
   ill-formed.

   If invoking f itself and invoking a copy of f behave differently, the behavior is
   undefined.
   4) The copy constructor is deleted, std::packaged_task is move-only.
   5) Constructs a std::packaged_task with the shared state and task formerly owned by
   rhs, leaving rhs with no shared state and a moved-from task.

.SH Parameters

   f   - the callable target \fI\fI(function\fP, member function, lambda expression, function\fP
         object) to execute
   a   - the allocator to use when storing the task
   rhs - the std::packaged_task to move from

.SH Exceptions

   2) Any exceptions thrown by copy/move constructor of f and possibly std::bad_alloc
   if the allocation fails.
   3) Any exceptions thrown by copy/move constructor of f and by the allocator's
   allocate function if memory allocation fails.

.SH Example


// Run this code

 #include <future>
 #include <iostream>
 #include <thread>

 int fib(int n)
 {
     if (n < 3)
         return 1;
     else
         return fib(n - 1) + fib(n - 2);
 }

 int main()
 {
     std::packaged_task<int(int)> fib_task(&fib);

     std::cout << "Starting task\\n";
     auto result = fib_task.get_future();
     std::thread t(std::move(fib_task), 42);

     std::cout << "Waiting for task to finish..." << std::endl;
     std::cout << result.get() << '\\n';

     std::cout << "Task complete\\n";
     t.join();
 }

.SH Output:

 Starting task
 Waiting for task to finish...
 267914296
 Task complete

   Defect reports

   The following behavior-changing defect reports were applied retroactively to
   previously published C++ standards.

      DR    Applied to              Behavior as published              Correct behavior
   LWG 2067 C++11      the parameter type of the copy constructor was  added const
                       packaged_task&
   LWG 2097 C++11      for overloads (2,3), F could be                 F is constrained
                       std::packaged_task<R(Args...)>
